For the record, I did pay a professionnal translator, and here is his feedback:
In mottoes it’s far more common to use an infinitive, ‘to hope for the best’, ‘to expect etc’, instead of saying it as an order. To me it makes more sense to say it as an order on a tattoo. I think it’s more personal. The first uses the infinitive, the second the imperative. The ‘s’ in exspecta can be left out if you prefer the look. It is possible to preserve the word play for ‘expect the unexpected. This would be mean changing subita to inex(s)pectata, although it makes the phrase quite a bit longer than the other two.
To make the third line more similar in length to the other two (which I try to aim for with tattoos) you can simply have spectare or specta, with ex- understood. This is a poetic licence, but perfectly acceptable.
Edit: I eventualy went with the following form:
For the record, it was for a tattoo: